In general, if a transmission distance for an optical fiber transmission path is formed to be long, a transmission speed is made to be high, and a number of a wavelength multiplication increases, there are a problem such as a transmission loss, an accumulated wavelength dispersion, and a polarization mode dispersion (Polarization Mode Dispersion, hereinafter called as “PMD” for short). It is possible to compensate the transmission loss by realizing an erbium-doped optical fiber amplifier. Next problem is an accumulated wavelength dispersion. A relationship between a transmission speed and an allowable wavelength dispersion is shown in FIG. 3. It is possible to compensate this accumulated wavelength dispersion by a module in which a dispersion-compensated optical fiber, etc. is used.
By the way, the S-SMF is commonly used all over the world presently. If a transmission for 1.55 μm bandwidth is performed by using this optical fiber network, a wavelength dispersion of approximately +17 ps/nm/km is generated in this 1.55 μm bandwidth. Therefore, if a signal is transmitted by using this optical fiber, the transmission characteristics deteriorates greatly by an influence of the accumulated wavelength dispersion in a long distance transmission. Also, although an NZ-DSF which has a smaller wavelength dispersion in 1.55 μm bandwidth than in the S-SMF is installed mainly for a long distance transmission path, it is necessary to compensate the wavelength dispersion which is accumulated on the transmission path.
Furthermore, at present, a wavelength multiplication transmission (Wavelength Division Multiplexing, hereinafter called as “WDM” for short) has been developed along with an increase in a transmission capacity; thus, the WDM transmission has already been realized practically in a lot of transmission paths. It is necessary to reduce the wavelength dispersion which is allowable over an entire wavelength bandwidth which is used in the WDM transmission. Therefore, it is necessary to compensate not only the wavelength dispersion but also a dispersion slope. A transmission optical fiber, a dispersion-compensated optical fiber, and a remaining dispersion characteristics after compensating the dispersion are shown in FIG. 4. A code for a dispersion slope of the dispersion-compensated optical fiber is opposite to a code for a dispersion slope for a transmission optical fiber, there is a case in which a remaining dispersion may be small in a wide range of bandwidth. An RDS is named for an index for indicating a performance for compensating this dispersion slope. The RDS (RDS; Relative Dispersion Slope) is a ratio of a dispersion slope with reference to the wavelength dispersion. RDS can be shown in a following formula (1) under condition that D indicates the wavelength dispersion and S indicates the dispersion slope.RDS=S/D  (1)
In order to reduce the remaining dispersion in a wide range of bandwidth, it is necessary to adapt a value which has an opposite code of a dispersion against the wavelength dispersion of the transmission optical fiber such that the RDS is as close as possible.
For such a dispersion-compensated optical fiber, for example, Japanese Unexamined Patent Application, First Publication No. Hei 6-11620 discloses a technology for a dispersion-compensated optical fiber which has a wavelength dispersion which is −20 ps/nm/km or lower for compensating the wavelength dispersion in 1.55 μm bandwidth in a standard single mode optical fiber which has a zero-dispersion wavelength in the wavelength 1.3 μm bandwidth. Also, Japanese Unexamined Patent Application, First Publication No. 11-95056 discloses a technology for a dispersion-compensated optical fiber in which a dispersion slope is reduce while reducing a connection loss such that an absolute value for the wavelength dispersion per a unit length is increased.
Also, Japanese Unexamined Patent Application, First Publication No. Hei 8-136758 discloses a technology for a dispersion-compensated optical fiber in which the dispersion slope is minus such that the wavelength dispersion is −100 ps/n/km or lower.
On the other hand, Japanese Unexamined Patent Application, First Publication No. Hei 8-54546 discloses a small-diameter dispersion-compensated optical fiber which has a first coating layer of which Young's modulus is 0.1 kgf/mm2 on an outer periphery of an optical fiber naked line (cladding) of which outer diameter is smaller than 125 μm and has a second coating layer of which Young's modulus is 150 kgf/mm2 or greater on an outer periphery of the first coating layer in an optical fiber which has a coating structure shown in FIG. 2. In a sixth embodiment of this Japanese Unexamined Patent Application, First Publication No. Hei 8-54546, a small-diameter dispersion-compensated optical fiber is disclosed which has 60 μm cladding, 160 μm coating, and −80.0 ps/nm/km.
Also, Japanese Unexamined Patent Application, First Publication No. Hei 10-115725 discloses a technology for a dispersion-compensated optical fiber which is connected to an optical fiber which has a wavelength dispersion in a transmission wavelength and compensates the above explained wavelength dispersion so as substantially to nullify the wavelength dispersion in an entire transmission system in the transmission wavelength. Also, Japanese Unexamined Patent Application, First Publication No. Hei 10-115725 discloses a technology for a dispersion-compensated optical fiber for reducing the wavelength dispersion in an entire transmission system in the transmission wavelength as close as zero which has a multi-layer structure such as at least two layers or more which comprises a core which is formed by a silica glass, a cladding which is formed by a silica glass on an outer periphery of the core, and a coating layer which is formed by a plastic resin member on an outer periphery of the cladding such that an outer diameter is smaller than 250 μm, the coating layer has 20 μm thickness or greater, and an outer layer which has a Young's modulus which is higher than that of an inner layer. A small-diameter dispersion-compensated optical fiber is disclosed in this Japanese Unexamined Patent Application, First Publication No. Hei 10-115725 and in embodiments in a specification of U.S. Pat. No. 5,887,104 such that a diameter of an outer cladding is 60 μm to 125 μm, an outer diameter of coating is 110 μm to 250 μm, and the wavelength dispersion is approximately −105 ps/nm/km.
Also, it is reported, in a B-13-4, p-585, thesis titled as “Small Dispersion-Compensated Optical Fiber For Compensating Dispersion”, 1999 General Conference, The Institute Of Electronics, Information And Communication Engineers, by Naoto Ogawa et al, that there is not a problem with regards to an increase in the loss due to a side pressure and rigidity point of view if a proto-type for a small-diameter dispersion-compensated optical fiber is produced in which a wavelength dispersion is −102 ps/nm/km to −110 ps/nm/km, an outer diameter of a cladding is 90 μm to 125 μm, and an outer diameter of coating layer is 150 μm to 185 μm.
Such a dispersion-compensated optical fiber is formed to be a module by disposing in a casing for winding a coil of which length is ⅕ to 1/7 of the transmission optical fiber. Although, when an accumulated dispersion in the transmission optical fiber of which length is approximately 120 km is compensated by a module of a dispersion-compensated optical fiber, a necessary length for the dispersion-compensated optical fiber is approximately 20 km; thus, it is necessary to wind a very long dispersion-compensated optical fiber.
However, it is preferable that a size of the module should be constant with regardless to a necessary amount of for compensating the dispersion. Even if an absolute amount of the dispersion which is compensated is great, it is necessary to wind the dispersion-compensated optical fiber as long as possible in a small casing. Furthermore, it is preferable that this module for the dispersion-compensated optical fiber should be as small as possible so as to form a small transmission device.
A volume of the module depends on a volume for winding the dispersion-compensated optical fiber. Such a winding volume is indicated by a product for a cross sectional area which includes a coating and a spiral length of the dispersion-compensated optical fiber.
Therefore, it is effective if dispersion-compensated optical fiber is formed such that its diameter should be as small as possible for forming a small module as disclosed in the Japanese Unexamined Patent Application, First Publication No. Hei 8-54546, Japanese Unexamined Patent Application, First Publication No. Hei 10-115725, or U.S. Pat. No. 5,887,104.
The development for the dispersion-compensated optical fiber has been made such that an absolute value for the wavelength dispersion per a unit length unless the transmission loss is deteriorated, and an optical characteristic and reliability are not deteriorated; thus, the diameter of the optical fiber should be as small as possible in an allowable range. However, the technology which are disclosed in the above mentioned publication and a report by an institute was not sufficient for realizing a small module while restricting an increase of a loss which is caused by a bending loss over an entire range of the usage wavelength.